I have the exact same issue. It's the only 2.5 I've got as a light switch and the only one (I have 10 others as covers/blinds) and the only weirdly behaving one.
Exact same behavior here, all other shelly 2.5 are using roller/shutter mode and the one set in relay mode is crapping out... if it works at one point just turning ON the lights via MQTT basically makes it not respond anymore (to further mqtt commands)
Interestingly enough it seems that most commands still seem to work in reasonable time frames like "/status" and even the "/relay/0?turn=on" (or off) commands don't show any delay...
There seem to be some small delays from time to time on those commands as well, so maybe there is some regular "high load" that interferes with tasks that take longer (like loaing the web interface)