There is more than one ISRG Root X1. There's the actual root certificate, valid until 2035:
Root certificates should never be sent explicitly, since clients already know them. Sending them explicitly just wastes bandwidth and does not increase compatibility.
Next, there's ISRG Root X1 cross-signed by DST Root CA X3:
This cross sign is included by default to enhance Android compatibility. The cross sign does expire next year. Therefore the plan is to phase it out starting early next year:
You should still be getting the cross sign though, since the above change should not have happened yet. However, it is possible to opt-in to the shorter chain for a few years now. Perhaps your ACME client explicitly requests the short chain?
Otherwise, it is also possible that web tools do not show the true chain send by server. To validate this we need to know your domain, so that we can check the chain using other tools.