mpdf: images are not shown in printed pdf with default syntax (and never in some servers)

• Error
• Regression
• Support request

some images are printed, but some others are not, and they seem to need to have no other params params than the minimum, however, using standard syntax from a brand new tiki site embeds wiki pages with a syntax that mpdf is not able to make it just work, and images are not shown in the pdf produced.

Reproduced here:
http://seeds4c.org/Ubuntu+16.04+LTS+for+Human+Beans
(you can produce the pdf by yourself there)

Code that mpdf doesn't seem to like:

{img src="display466" link="display466" width="400" rel="box[g]" imalign="center" desc="Click to expand" align="center" styleimage="border"}
{img fileId="60" thumb="box"}

Code that mpdf seems to like:

{img src="https://www.hecticgeek.com/wp-content/uploads/2012/09/Editing-the-disable_wol-script-in-Ubuntu-12.04.jpg"}
{img src="display550" width=600}

(removed the examples with params like src="dl437&display" since they were written by the time when tiki accepted that oldish syntax by mistake)

Update:
In a production server in our work, no images are shown in the pdf at all, with neither syntax. Is there any log file with potential information on why no images are shown in the pdf produced by mpdf?
I confirm that we have php7.0.* and php modules gd, & mbstring installed server side.

Xavier

I removed "19.x" categorization because branch 19 doesn't exist yet. The category "regression from version 18 to 19" is accurate here and already assigned.

Update answering luci (Why can't standard users reply to comments, such as my user "xavi" wit no admin rights? 😕 ). Answering here instead (lacking time):

• replacing that old syntax with new and valid syntax (? intead of &) still yields some broken images in the pdf produced by mpdf. I granted access to amnabilal to a server where she can reproduce. I didn't hear any more feedback from her about it, but I confirm that this issue still exists in several servers of mine, with different setups.

Update: Default syntax with param thumb="box" keeps images away from the printed pdf (even with valid https cert and fully valid params aside of mpdf)
Example:

{img fileId="60" thumb="box"}

Issue with thumb="box" or thumb="zoombox" for instance, confirmed still in 21.5vcs as of today

1. 2018 06 24 093455

This bug has been demonstrated on show2.tiki.org

Please demonstrate your bug on show2.tiki.org

This bug has been demonstrated on show.tikiwiki.org

Please demonstrate your bug on show.tikiwiki.org