Between versions 1.14.3 pre1 and 1.14.3 pre2 mouse reacts a lot more sensitive. Either in a fresh 1.14.3 pre2 world or converted from pre1.

I only noticed when upgrading to 1.14.3 since I usually only play full releases. Then I did some tests and found the issue to be occuring since 1.14.3 pre2.

More people reported this when 1.14.3 launched:

https://www.reddit.com/r/Minecraft/comments/c4pjj8/minecraft_java_edition_1143/

https://www.reddit.com/r/Minecraft/comments/c5bmbv/sensitivity_change_in_1143/

https://www.reddit.com/r/Minecraft/comments/c4qgbh/experiencing_mouse_sensitivity_issues_on_java/

Might be related to lwjgl, see https://bugs.mojang.com/browse/MC-144107