[MCM-430] [BLOCKING] Mob spawners created in earlier versions are now non functional.

Partner Rank:

Partner Team Reporting:

Jigarbov Productions

Verification builds and Platforms:

ver. 1.8.0.13, build 1327420, Win10

Summary:

Mob spawners made in 1.8, 1.7, 1.6 and maybe earlier builds no longer function in this beta build.

Impact:

Every map released since 1.6, maybe earlier that uses mob spawners either as a mechanic or selling feature is rendered broken or unplayable. I estimate in the order of millions of items in childrens catalogs will be affected by this.
My personal maps effected:
Minecon Earth
Papercraft Adventure
Infinity Plumber
Lucky Present Survival - Free content for christmas promotion.
Solar 2
etc.

Repro Steps:

Download the content "spawners don't work" from this here bug report
Open it
Observe the area where the mobs should be spawning

Observed Results:

The spawner mobs spinning around being all like "I'm going to spawn!" but then not spawning anything. Forever.

Expected Results:

The spawners should spawn things.

Screenshots/Videos attached: No

It would literally be a screenshot of an empty room... wouldn't be useful. But I did attach a world made in 1.6 with some spawners both naturally spawned and made with a mob egg.

Regression Builds:

1.8.0.10 Beta 27 1279697

Notes:

Reported over on the main Jira too, so the kids are noticing! Think of the children! https://bugs.mojang.com/browse/MCPE-38706