I'm not sure if this is normal and I can't seem to find a setting for it but when players use the nether portal with the sign on it they are set on fire upon teleporting.

If this is still an issue remake it here https://github.com/OpticFusion1/Multiverse-SignPortals/issues

Unable to replicate. It could be your TO portal's location that is on fire?
Closing ticket because it's years old & unable to be reproduced. Update to the latest versions of Multiverse (Core, Portals and any others you may use). Please open a new ticket if this issue persists or for a faster response, you can also receive support on the Multiverse Discord.