So, I'd like to post an update on this S19j Pro+ situation.
We were initially skeptical of the soldered heatsinks that are very similar to those on Antminer S17 series.
Call it a case of once bitten twice shy.

We've run many of these S19j Pro+ units in the mine through this summer, including some with substantial overclocking of upto 135T.
The soldered heatsinks seems to be doing a much better job of heat dissipation and S19j Pro+ miners consistently ran about 5-10 deg C cooler than other S19 series miner in the mine.
No, hashboard issues this summer either. While this is with just a few months of testing, I'd say its safe to assume that whatever issues Bitmain had with soldered heatsinks on S17 have now been fixed with S19j Pro+.

Someone mentioned it was improper chip tinning which caused issues with S17 series and not the soldering itself, so it's possible that Bitmain has improved its chip tinning process.