I raised this bug some time ago
https://fiddle.sencha.com/#fiddle/2k99
It's rather problematic as it occurs in a few different use cases - the fiddle just demonstrates one of them
It's recorded as bug EXTJS-27428 and it's quite serious but the "Remains of Sencha" have not seen fit to fix it yet - and possibly never will.
Any of the Sencha experts on here have a suitable workaround please?