Just after the war in France Thomas Holland became rich enough to sue Montagu, which is why he emprisons her now. Since Montagu was the King of the Isle of Man, it is very plausible that she was imprisoned here at this exact time period and it is very plausible that Thomas Holland found a way to sneak her out to testify. Either way she testified and her marriage with Montagu was cancelled. https://en.wikipedia.org/wiki/Joan_of_Kent#Early_marriages

All documents about the Isle of Man from 1333 to 1390 are lost ! So if such a thing did happen, it would have disappeared from history anyway. Also all parties involved would have a strong incentive to not talk about it.