I added image fields in a custom taxonomy. And to my surprise they are all saved in the options using the key “taxonomyslug_termid_fieldname”.
Is this a standard wordpress feature or acf’s feature? o.O
This is how ACF has always worked, ACF was saving values this way before term meta existed in WP. The developer of ACF plans on making the change to using term meta in a future version. (maybe 5.5. or 5.6). Until then there are some workarounds in this topic https://support.advancedcustomfields.com/forums/topic/how-to-use-wp-term-meta-on-acf-the-easy-way/