Windows 10, java build 1.8.0_211-b12
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:
Might be related to lwjgl, see https://bugs.mojang.com/browse/MC-144107
MC-155147 Mouse acceleration with the new 1.14.3 update