the_modified_time showing wrong date

I have an wordpress self-hosted installation that is over 10 years old. I was updating the template files to show the last modified date instead of the published date for my posts.

Old snippet: time datetime=?php the_time('c');? ?php the_time('j F Y'); ?/time

New snippet: time datetime=?php the_modified_time('c');??php the_modified_time('j F Y'); ?

Here’s where it got weird. Every post that was modified before 6 februari 2021 shows the date 6 februari 2021. Every post that got updated after 6 februari 2021 shows the correct date.

I double checked this. There are posts that haven’t been updated since 2014 or 2011, yet they show 6 februari 2021. I even went into phpMyAdmin and the post_modified field shows the correct date (not 6 februari 2021).

Does anyone have an idea what it causing this or how to fix this? I have some suspects:

• I’m using older php version: 7.2.34
• I’m using an old sql server (WP site diagnose says so)
• I vagely remember doing a sort of database update. Yoast SEO promted me to do something for indexing purposes.