Can't add a second billing model to a printer

[eduardomozart]

• Version of the plugin : 1.7.0

• Version of your GLPI : 9.5.2

• Steps to reproduce (which actions have you made) :
  After adding a billing model to a printer, it's not possible to add a second billing model to the printer because it is not shown at the "Linked items" tab of the second billing model. The wiki states that it's possible: "Il peut exister plusieurs modèles de facturation pour une imprimante.".

In the screenshot above, the printer "SEC001599A9C2CE" is linked to one billing model.

When trying to link it to a second one, it didn't show up as available into the second one:

It didn't work into the "Billing model" page from the printer (Assets > Printers > [Print name]) neither.

The only way to add a second billing model is through the massive action "Printercounters - Set billing model" at Assets > Printers > Action menu, so it works normally:

• Expected result :

It should be possible to add a second billing model into the "Billing model" printer page and at "Billing models" at the plug-in page.

• Actual result :

There's no error at 'php-errors.log' and 'sql-error.log', Apache logs neither.

• URL of the page :

• Screenshot of the problem (if pertinent) :