The break-in period for tubes is real. At least 20hrs. but for true sonic improvement even more like 50 hrs or so depending on brand and tube.
If the tubes are new and not burnt-in before testing/matching them they will lose their closely matched condition after 20 hrs. The operation point and the plate voltage of the amp in which the tubes are used is an even more important factor for matching and how they will finally sound. Tubes should be matched for the plate voltage and quiescent current of the actual amp.Some companies do that (upscale audio as far as I know). Adjusting the bias after a while (if possible) is important.