Well, given the fun times I had with high balances on many credit cards leading into the credit crisis, and the years I've spent paying them down with CLD from every lender except CapOne on the same $10k limit, 10.9% APR, and thus the stability I've experienced with CapOne, I would say that, yes, a surprise $50k open amount on your credit report (I'm presuming it did report on your CR) may have given them pause on one of their SP. 
But that's just me speculating.
@Anonymous wrote:
I think you need to call cap one EO tomorrow, explain you situation and get that account back open.
After the mortgage, app for amex to backdate your member since date and burn that cap one card.
Always remember what cap one did to you and diversify.
This x1000; you absolutely do not want that card closed while going through the mortgage process - all AU accounts has been known to get you kicked out of a mortgage previously and this is a major change if your credit gets pulled again prior to closing and the fact she's still your fiance (no homemaker exception as was cried about when FICO 8 was released) though I don't *think* it has to be open tradelines for mortgage underwriting, just have to have some personal history; however I just don't know mortgage underwriting well enough to state that with any accuracy.
FICO 04/98 won't kill you for the account being closed like like a FICO 8 score would so that's one advantage to the GSE's quaintness regarding scoring models, but say you're sorry, say it again, say it like you mean it, and hope Cap 1 restores the account. If you can't, ugh. I'd probably go and talk to the LO right after going through the CSR / EO two step with Cap 1, I wouldn't want them to be surprised by that mid-process if your credit gets pulled again which is routine during the mortgage process.
And go get 3 cards of your own after you close on the house .
