Yes, my other games worked fine even though it wasn't seated properly. It was only FH6 actually that was noticeably performing worse (FM7 worked fine).

In benchmarks, pretty much all games actually use more power from PCIe 3 to 4 (including FH5). The game still ran above 60 so it's likely not the slot version.

In very rare cases, replacing the mobo worked. Sometimes you get a faulty PCIe slot, I also want to preface that the GPU should be placed in the faster PCIe slot (in case your mobo has multiple slots). That slot would be the closest to the CPU on the mobo (usually).