Hi odedp,
subdomain addressing should work the same way as plus addressing. When you say "it fails" you mean the mail isn't sorted into the folder it is supposed to be or doesn't the mail arrive at all?
In general, when using subdomain addressing, your 'anything' must not contain a dot 'any.thing' or an underscore 'any_thing' according to
https://www.fastmail.com/help/receive/addressing.html
I use subdomain addressing a lot and don't have any issues.