Fortunately it wasn't something catastrophic. I would definitely pull that tube before something bad happens. I have had fairly good luck with other GL tube types but I did have a KT 66 red plate and a KT77 lose vacuum.
I recently have been paying the match and low noise fee on new tube purchases , so far so good . I have a tester but I feel more comfortable if they are matched on something like an Amplitrex or similar .