Steam has own runtimes which you can try. You’ll have to run the software from Steam & probably force the runtime in the properties of the application (the procedure is the same as with Proton):
Right click on the application → Properties → Compatibility → Force the use of a specific Steam Play compatibility tool.
Select Steam Linux Runtime 3.0 (sniper) or 1.0 (scout) (at least 3.0 seems to provide libtiff.so.5).
If this doesn’t work you could try:
- Report the issue to Valve (there should be an issue tracker on GitHub)
- Build an Flatpak extension which provides the needed dependencies
- Take the library from another distribution (beware: foreign libraries can be incompatible)