When traffic other than HTTP/S (eg: SSH traffic, etc...) traverses the FortiGate in version below 6.2.5 and below 6.4.2 on port 80/443, it is not redirected to the transparent proxy policy for processing, as it doesn't have a valid HTTP header.
https://fortiguard.com/advisory/FG-IR-20-172
Source: Mitre, NVD
Published: 2021-03-04
Updated: 2021-03-11
Base Score: 4.3
Vector: CVSS2#AV:N/AC:M/Au:N/C:N/I:P/A:N
Severity: Medium
Base Score: 7.5
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Severity: High