Websocket connection one event behind

[Sp1l]

Apache 2.4.63
mod_http2 2.0.31
mod_tls 0.14.0
rustls 0.14.0 and 0.15.0 (both tested)

Website seems to work fine otherwise, but has a problem with one app. It is a terminal via a web-UI using python and term.js. It uses websockets.

Running the same config with mod_ssl and OpenSSL 3.x works fine. When using mod_tls, every keystroke is hidden. Upon hitting the next key, the character appears.

<Location "/webcli/">
    AuthType Digest
    AuthName "webcli"
    AuthUserFile "htdigest.user"
    Require valid-user
    ProxyPreserveHost on
    ProxyPass        http://127.0.0.1:8080/webcli/ upgrade=websocket
    ProxyPassReverse http://127.0.0.1:8080/webcli/
</Location>

First time ever I'm trying to debug some webapp using websockets, tips appreciated!