Data lost by not longer prefilled multilang text fields in the repeater context (3.0.246)

[skeltern]

A client discovered a strange issue in 3.0.246. Setup:

• Multilang is active and 2 or more languages are active
• A Repeater Matrix item contains anyone default repeater
• Multilang text fields (input, RTE) in default repeater

After fresh CMS login it seems to be fine. All multilang fields
are prefilled with stored inputs to edit (default behaviour).

But after a page save, multilang values got saved but only the default
language is still prefilled. Each other multilang inputs are no longer prefilled and
just empty. That means data lost is very easy possible just by saving anyone page.

I can't reproduce the issue in 3.0.229, but always in 3.0.246.
On 5+ websites including my own. It seems to be a bug with prefilling
some multilang fields by cvontext (missed value="saved input").

The chance to lost data is high. Does anyone know a quick workaround?

[skeltern]

Update. 

After lots of testing. It seems that described issue affects
only older/existent repeaters. Not fresh created in 3.0.246/247.

I can create a new simple multilang input field, connected with two different
repeater items inside a Repeater Matrix. First repeater created maybe
two years ago. Second has same settings but created today.

After page save first don't provide any language values anymore,
second works as aspected.

The question is, what's the difference here. And how could I trigger or
update existent and filled repeaters to fix this issue. I tried to clean up
repeaters under "Storage" and also removed all DB "caches". Without
luck so far.

Any idea would be helpful.