For now, you can work around it by using Cmd+0 (Mac) or Alt+0 (others) to set the style.
What happened:
Last year, we floated a proposal internally to tweak the behavior of named style application. In 11.1 (and for years earlier), if you type, say, Alt+7 (for Text style) in an empty cell, it converts the entire cell to Text. But if you do it in a non-empty cell, it only converts the things you are about to type to a Text StyleBox
. The proposal was to make it consistent so that, at any point, if you have a blinking caret selection in a cell and apply a named style, it applies to the whole cell.
This would be an inconvenience to few cases where people might wish to apply named styles as they're typing inline, but it would be a convenience in other cases where one has accidentally started typing several characters or words into a cell without realizing that the wrong style had been selected. The proposal included the possibility of selecting a range of text and applying a style to that range specifically just as always had been possible, so the legacy usage would still have an easy way to be applied.
Unfortunately, we made a mistake and ended up causing named style application to always promote to the entire cell even if you have a range of text selected. This is already resolved in our internal nightly build, but did not get resolved in time for 11.2. My apologies for that.
In the mean time, the same change didn't get applied to the "Other..." style dialog, and so you can use that as a workaround.