Just a small QoL feature I'm not sure whether it's unimplemented or broken.
I'm aware that when you have an Anime set as "Watching" on your list, and you mark the last episode as watched (via the + button at least) said Anime will now be marked as "Completed". However, when you have an Anime set as "Planning", and you marked the first episode as watched the category isn't updated to "Watching". Could there be some issue preventing this from happening, or is it just something not yet implemented?
Coincidentally, I happened to make a thread saying pretty much the same thing 9 hours before you did. https://anilist.co/forum/thread/5453
As long as something like that could be opt-out, I wouldn't mind it. I use the eps watched as an indicator of whether I have already obtained the means to watch those anime series, with 0 eps seen being a no and 1 ep seen being marked as a yes, even though I haven't watched the first ep yet. If I were to actually watch one ep and then cut it there, I would move it either to on-hold or dropped, depending on the reason