Merge features: manual value is not respected

[MMR100]

In QGIS 3.14, when merging selected features, if I double click on one cell in "merge" line, I can change the value manually.
Then automatically the dropdown list switch to "manual value" instead of "feature XXX".
However, after merging, the result is not the manual value written but "feature XXX" value.

If I do the same process but this time I deliberately click on the dropdown list and select "manual value", then type my value, the result will be the value typed as expected.

[gioman]

If I do the same process but this time I deliberately click on the dropdown list and select "manual value", then type my value, the result will be the value typed as expected.

@MMR100 on QGIS master on Linux not even this, it still writes the value from one of the original features. I believe this is a regression and at high priority bug as it can lead to wrong/dangerous results in an unexpected way.

[DelazJ]

I confirm with 3.14.15 on macOS but like @gioman, even when I deliberately select "manual" in the drop-down, I still get the first feature value.

[azetagis]

Also in QGIS LTR 3.10.9 on win7 32, like @DelazJ even when I deliberately select "manual" in the drop-down, I still get the first feature value.