Cart not emptied after checkout

Hi,

since a few versions we have a weird and random problem, when an order is placed the cart seems emptied on the “thank you” page, but as soon as another page (and cart fragments) is called, the items just ordered reappear in cart.

Server is OpenLitespeed with PHP 7.4
LScache is enabled
Redis is enabled
OPCache is enabled
WP 5.6.1
WC 5.0.0 and 4.9.2
ATUM 1.8.5

/*** What we already tried (with no success): ***/
– if we disable ATUM plugin the wrong behaviour immediately disappears
– we disabled all plugins, including LSCache, and only kept Woocommerce and ATUM. No success. Again, as soon as we disable ATUM the problem disappears.
– we tried with two themes, quite an old one (VG Donnan) that works since years on our woocommerce, and a brand new installation of Blocksy on a test server.
– tried to clean all transients, both expired and not expired.
– tried to clean all browser cache, including cookies and local storage
– apparently the problem does not appear to all customers (we have thousands), but only in an apparently random way…
– We do NOT have Cloudflare enabled
– We disabled PHP OPcache and Redis
– We installed old versions of ATUM down to 1.7.9 (we do not remember exactly since when the problem started…)
– The PHP logs do not show any particular error
– The cart cookies are correctly removed when order is placed but immediately reappear as soon as another page is loaded (probably by cart-fragments?)

It’s quite urgent as we hve more than 10.000 products, and to manage them with only the normal products list on Woocommerce it’s almost undoable.

Thanks in advance!