Taxonomy: array_walk() expects parameter 1

In a site I’m currently developing, I define the following kind of frontmatter:

title: ?Alos - Matrice
artwork: alos_matrice.jpg
artist: [?Alos]
format: [LP, Digital]
category: [Doom, Electronic, Experimental]

I don’t know starting which version the problem has appeared but it looks like one of the last updates has introduced a bug (?) related to taxonomy handling. The current version I’m using is 1.6.14.

=> If there’s no space between a taxonomy type and its value(s) e.g.

It causes the following error:
array_walk() expects parameter 1 to be array, string given (…\system\src\Grav\Common\Page\Page.php 2308)

Is it normal behaviour? I didn’t had this problem before updating Grav.