This is caused by the following backward incompatible change in GNU 4.3: https://lists.gnu.org/archive/html/info-gnu/2020-01/msg00004.html
* WARNING: Backward-incompatibility! Previously appending using '+=' to an empty variable would result in a value starting with a space. Now the initial space is only added if the variable already contains some value. Similarly, appending an empty string does not add a trailing space.
