No, this is for the zap menu that comes up when you long press zap button on a note.
#[2] what do you expect to happen when you clear the value from the custom zap amount field, go back, and return to the custom zap amount view? Zap is downstream of this setting.
I am able to recreate behavior where the first digit only is retained on clear. https://nostr.build/av/nostr.build_143bf469a9151ad51774ad84913bfd60c52d0080e44fa0fea475c306c3eb60bd.mov
Discussion
Although, it seems to also apply to the zap settings as you are showing.