Add if missing, what it does is it adds a VLAN tag on egress, and is used on Trunk ports… So you were adding a VLAN tag…
The leave as is on the other hand, it does not add but it does not remove as well a VLAN tag on egress, it is used for hybrid ports… It is the default value…
Always strip, removes the VLAN tag from egress and is used on access ports…
Read here https://wiki.mikrotik.com/wiki/Manual:Switch_Chip_Features#Features