When I run this on my system, the API presents the following certificates which get verified correctly:
depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = R3
verify return:1
depth=0 CN = api.snapcraft.io
verify return:1
---
Certificate chain
0 s:CN = api.snapcraft.io
i:C = US, O = Let's Encrypt, CN = R3
1 s:C = US, O = Let's Encrypt, CN = R3
i:O = Digital Signature Trust Co., CN = DST Root CA X3
---
Maybe you’re running through a proxy?