It's not a bug - But a common issue with a simple solution:
Enable the Cutoff checkbox for all instruments that share the same channel, and move the cutoff slider to the far right until it says "Off".
Another solution (but not compatible with Impulse Tracker, thus just recommended for MPTM modules) is to use the "local" filter mode, which will reset the cutoff and reso values on every note
on a channel (and will thus probably generate unwanted results when using Zxx). This can be enabled by putting a S9D effect on this channel. The lowpass filter can also be reset manually by putting a Z7F effect between to notes, but that just sounds right if no note is currently playing on that channel, of course.
It is not a temporary thingy, cus that would be glitchy
Why would it be "glitchy"? That's exactly the difference between S9C (global filters) and S9D (local filters), and both modes make sense.