i just had it occur a minute ago, i took a quick video using my phone and uploaded it to youtube as a unlisted video, https://youtu.be/MqeW4__Pohc ,
it has been happening since the beginning of 1.16 snapshots and the combat test snapshots and maybe since the beginning of 1.15 snapshots, i have not played the 1.15 release or 1.14 release long enough to have notice this bug on those versions yet.
Device is iPad 10.5 with iPadOS13.2 / iOS13.2 with a steelseries nimbus controller (MFi with Xbox like layout with no clickable thumbsticks)
Fishing line not connecting and bobber floating in the air is still present in Minecraft (java) 1.10.2
added screenshot of it. (river/squid photo)
i am on snapshot 20w12a, and i keep having the shield become unusable until i unequipped and reequip the shield, the quickest way is tapping F twice to move it to main hand then back to offhand, i notice it happening the quickest/most frequent during building/digging projects (i have been changing hotbar slots with mouse wheel and click and dragging tools to combine repair them and make new tools at the crafting bench and accessing furnaces and chests and digging and placing blocks, basically digging/building an underground base underground using stone bricks, seems to occur once every 2-10 minutes doing the above actions).
edit: grammer and additional info