Have you read up on Linux’s DSA documentation Total obsolete VLAN documentation - #14 by anon82920800 - General discussion - Turris forum ?
That aside there are some bugs in Linux’s DSA code that can cause issues and are being fixed mostly in Kernel 5.4 and up and not all of it gets backported to kernel 4.14 (OpenWrt 19.7.x | TOS5.x), albeit the TOS dev already implemented some patching on their own in TOS5.x