It turns out that mandate does indeed demand that its complement content clause be subjunctive (i.e., mandate that there be). In the COCA, there are 38 hits for verb mandate followed by is, but not one of them is in a complement clause and every mandate is a participle, for example:
- One energy conservation measure now essentially mandated by law is use of smaller cars (60 days).
- this would set a bad precedent for mandating a vaccine which is not spread through casual contact
- This mandates that responsible engineers be employed by the city.
- ... mandating that the snakes be released after the hunt at the place of capture
- the program is mandated to be cost neutral
- it is important that assessments be used only for those purposes for which they were designed
- it's important that there be accountability
- for the U.S. role to be effectively played, I believe it is very important that there is coordination and agreement
- It's important that any changes are implemented to ensure that change doesn't cause distress.