Well, multi monitors support is quite a nightmare in Budgie.
See
⋅ Unable do get full screen windows on secondaries monitor
⋅ Can't use whole screen on monitor which comes after plank
Did you change recently position of the dock ( plank ) or the bottom panel ?
I’d say
⋅ try with plank on far left edge of the biggest screen, on the left,
⋅ and have that left bigger screen as primary monitor ?
By toying with panel and dock positions, and which screen is primary, you should find a setup that works for your workflow, more or less…