According to Tracking HTTP API specification

action_name (recommended) — The title of the action being tracked. It is possible to use slashes / to set one or several categories for this action. For example, Help / Feedback will create the Action Feedback in the category Help.

the setting several categorizes for actions worked well before upgrade to 3.8.
After automatic upgrade , the categories appears as one line in the reports

For example
Prior 3.8 upgrade 4 categories INSTALL/TAP/NOK/E_ENGINE_ALREADY_INSTALLED.
would appear as tree view hierarchy

Now both forms appearing in a report

• a tree view hierarchy
• 4 category in one line . separated by slashes ( probably new tracking reports )

(see attached picture)

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Matomo version: 3.8.0
MySQL version: 5.5.56-MariaDB
PHP version: 7.0.22