Undercroft duration oddities?

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Had some strange things happen yesterday while playing Duviri Experience.

First run I had Gara Prime with one of my own configs and when I used her in the Undercroft, she worked as expected at first, i.e. casting Mass Vitrify reset duration on Splinterstorm (and shattering it with her 1 added damage to Splinterstorm), but after a short while it stopped working, and I had to resort to using Splinterstorm purely for damage reduction since I couldn't add damage or duration to it.
Same thing in the second Undercroft of that run.

Next run I had Valkyr Prime, also with one of my own configs, with the Eternal War augment. That also worked at first, but then, even though I killed every single enemy with melee, very little duration was added (maybe once for every 15-20 enemies), and I had to just trigger the ability when it ran out.
Same thing in the second Undercroft.

Both frames worked like they should the day before yesterday.

Has anybody had something similar happen?