I couldn't replicate any of the issues in this thread. Neither weapon would show up when locked and the 30 cap Wired Lance that I increased to 35 cap via 5 copies of the same weapon (the Light element one in the pic) was exchanged without issue.
These types of things are logged in your Documents\SEGA\PHANTASYSTARONLINE2_NA_STEAM\log folder, specifically, the ActionLog file. Here's the one that I exchanged in the pic for comparison:
2020-10-20T09:50:25 60 [Recycle(Receive)] 10073289 Neek Bomber Golem Stead+9 attr:Light(60)
2020-10-20T09:50:25 61 [Recycle(Send)] 10073289 Neek Bomber Golem Barun+9 attr:Light(60)
2020-10-20T09:50:25 62 [Recycle(SendWarehouse15)] 10073289 Neek Chaos Malossistone Num(50)
(Note: if you don't have material storage then the Chaos Malossistone line will look different)
It doesn't show grind caps but it might provide you a bit of context as to what might've happened before and after the exchange.
Thanks for the info on where the logs were saved. I've edited my previous posts because apparently it was my fault things got screwed up. My apologies. Never try to upgrade weapons at 4am in the morning. Things are working as intended, on my end at least, so carry on.