Just looking into this now - this is a simple test page containing one jpeg image (and its webp copy in the same dir). There's no favicon though the browser looks for it, that's why it appears in the logs below.
So, with both rewrite rules mentioned above, this is all I get in the log:
2020-06-06 11:51:05.116635 [INFO] [178.20.203.17:50986:HTTP2-15#depth.gallery] [REWRITE] strip base: '/' from URI: '/'
2020-06-06 11:51:05.116720 [INFO] [178.20.203.17:50986:HTTP2-15#depth.gallery] [REWRITE] Rule: Match '' with pattern '^/?(.*)', result: 2
2020-06-06 11:51:05.116740 [INFO] [178.20.203.17:50986:HTTP2-15#depth.gallery] [REWRITE] Cond: Compare 'on' with pattern 'on', result: 0
2020-06-06 11:51:05.116744 [INFO] [178.20.203.17:50986:HTTP2-15#depth.gallery] [REWRITE] Rule: Match '' with pattern '(.+)\.(jpe?g|png)$', result: -1
2020-06-06 11:51:05.461945 [INFO] [178.20.203.17:50986:HTTP2-19#depth.gallery] [REWRITE] strip base: '/' from URI: '/favicon.ico'
2020-06-06 11:51:05.461982 [INFO] [178.20.203.17:50986:HTTP2-19#depth.gallery] [REWRITE] Rule: Match 'favicon.ico' with pattern '^/?(.*)', result: 2
2020-06-06 11:51:05.461988 [INFO] [178.20.203.17:50986:HTTP2-19#depth.gallery] [REWRITE] Cond: Compare 'on' with pattern 'on', result: 0
2020-06-06 11:51:05.461993 [INFO] [178.20.203.17:50986:HTTP2-19#depth.gallery] [REWRITE] Rule: Match 'favicon.ico' with pattern '(.+)\.(jpe?g|png)$', result: -1
If I delete the rule which adds the Cache-Control headers to all images, I'm now seeing the webp rewrite in action:
2020-06-06 11:52:17.848713 [INFO] [178.20.203.17:50997:HTTP2-15#depth.gallery] [REWRITE] Rule: Match '' with pattern '^/?(.*)', result: 2
2020-06-06 11:52:17.848719 [INFO] [178.20.203.17:50997:HTTP2-15#depth.gallery] [REWRITE] Cond: Compare 'on' with pattern 'on', result: 0
2020-06-06 11:52:17.848723 [INFO] [178.20.203.17:50997:HTTP2-15#depth.gallery] [REWRITE] Rule: Match '' with pattern '(.+)\.(jpe?g|png)$', result: -1
2020-06-06 11:52:18.058733 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] strip base: '/' from URI: '/guitars-bg-mob.jpg'
2020-06-06 11:52:18.058780 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] Rule: Match 'guitars-bg-mob.jpg' with pattern '^/?(.*)', result: 2
2020-06-06 11:52:18.058786 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] Cond: Compare 'on' with pattern 'on', result: 0
2020-06-06 11:52:18.058791 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] Rule: Match 'guitars-bg-mob.jpg' with pattern '(.+)\.(jpe?g|png)$', result: 3
2020-06-06 11:52:18.058795 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] Cond: Match 'image/webp,*/*' with pattern 'image/webp', result: 1
2020-06-06 11:52:18.058813 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] Cond: test '/home/depth.gallery/public_html/guitars-bg-mob.webp' with pattern '-f', result: 0
2020-06-06 11:52:18.058839 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] add ENV: 'accept:1'
2020-06-06 11:52:18.058851 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] Source URI: 'guitars-bg-mob.jpg' => Result URI: 'guitars-bg-mob.webp'
2020-06-06 11:52:18.058855 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] set forced type: 'image/webp'
2020-06-06 11:52:18.058858 [INFO] [178.20.203.17:50997:HTTP2-17#depth.gallery] [REWRITE] prepend rewrite base: '/', final URI: '/guitars-bg-mob.webp'
2020-06-06 11:52:18.192336 [INFO] [178.20.203.17:50997:HTTP2-19#depth.gallery] [REWRITE] strip base: '/' from URI: '/favicon.ico'
2020-06-06 11:52:18.192377 [INFO] [178.20.203.17:50997:HTTP2-19#depth.gallery] [REWRITE] Rule: Match 'favicon.ico' with pattern '^/?(.*)', result: 2
2020-06-06 11:52:18.192383 [INFO] [178.20.203.17:50997:HTTP2-19#depth.gallery] [REWRITE] Cond: Compare 'on' with pattern 'on', result: 0
2020-06-06 11:52:18.192387 [INFO] [178.20.203.17:50997:HTTP2-19#depth.gallery] [REWRITE] Rule: Match 'favicon.ico' with pattern '(.+)\.(jpe?g|png)$', result: -1
I'm not sure what to make of this other than, yeah, it's not applying the .webp rewrite rule. Can anyone gather any info from this?