MCPE-50258 is reported for Android. This one is for Windows 10.
Impact of this bug: Will affect all HD texture pack creators as they need to reload edits constantly - as in every few minutes to view their progress. This includes texture packs for Bedrock as they are all created in Java first, then ported - due to the lack of keystroke for reloading texture packs without restarting Bedrock client or exiting world.
It took about 5 reloads with Chroma Hills to bring my FPS to 1. But higher res packs are effected more. A 16x pack lost 1/2 FPS after 10 reloads. Lithos 32x lost about 10 FPS after 10 reloads.
Which texture pack?
The pack is Chroma Hills HD. I've tested it with ZigZag and PureBDcraft and the lines do not appear. This did not happen with 1.7 with the same pack.
This bug is also present in Windows 10 version, not just Android as reported here.