I think it's recursion anyway.

Crash log: https://pastebin.com/W6U5fzX8

Damaging a mob using a tool with Portly Gentleman then immediately capturing the same mob with the same tool results in a crash. The mob receiving damage from another source between the initial damage and the capture results in no crash. The damage source being a held arrow or bolt with Portly Gentleman did not result in the crash either.

Landmaster/PlusTiC#165 details the discovery and narrowing of the bug to an interaction between PlusTiC and Tinkers Tool Leveling, though I didn't realize at the time that it could be on TTL's side. I only realized recently that TTL could be and is what stores the latest damage source as NBT data on a mob. That issue goes into more detail than is probably necessary, though at the time I didn't realize that.

Tinkers Tool Leveling: 1.0.3
PlusTiC: 6.4.0.0