I am having the same problem with FrameTicks in 14.1 breaking something from 14.0 for custom tick labels.
dat = {{#, 6, 30}, RandomInteger[{10, 20}]} & /@ Range[1990, 2020, 1];
DateListPlot[dat,
FrameTicks -> {{All, All}, {None, {{{2005, 6, 30}, "X"}}}}]
Can someone with access to 14.2 builds see if this has been fixed?