The game crashes instead of displaying Forge's "missing/outdated dependencies" screen, with the following crash log. (doesn't get written to latest.log but MultiMC manages to catch it)

Reproduction on Forge 40.0.52 using Botania since it needs Patchouli and Curios:

• Botania-1.18.2-430 + curios-forge-1.18.2-5.0.6.3 (missing Patchouli): missing dependency screen as expected
• Botania-1.18.2-430 + curios-forge-1.18.2-5.0.6.3 + better-loading-screen-1.4.0: crash (same log as above)
• Botania-1.18.2-430 + curios-forge-1.18.2-5.0.6.3 + Patchouli-1.18.2-67 + better-loading-screen-1.4.0: loads and works as expected

Same, i just spend two full days troubleshooting this, but I also had Hammerlib which causes the similar crash so I didn't notice it at first.