hi chris!
ofc i realise it's because of the beta :)
just to let you know saving (even a new tw 5.3.3 initiated from quine) is unstable when running ios18.0beta on iphone 15 pro
my old 5.3.1 tw saves sometimes and not others
the 5.3.1 tw save still works fine on rclone serve webdav still on a mac (haven't put any macos betas on that mac, but have tested safari technology preview Release 196 (Safari 17.4, WebKit 19619.1.16.3) which works fine with rclone serve webdav to save still)
so it looks like this issue is a ios18.0beta issue (possibly something to do with safari 18 beta (20619.1.15)); hopefully upcoming ios betas might solve this
warmest wishes, maki