Python. Can't decode bytes from base64 to JSON

Question

I have a packages encoded by URL-encoded unpadded base64. It's looks like this:

b'DbMG_38MBgb8ru_RlDE1'

So I have a simple code which must turn this JSON bytes to Python object

import base64, json
b = b'DbMG_38BBgaI0Kv6kzGK'
a = base64.urlsafe_b64decode(b)
c = json.loads(a)

From b'DbMG_38BBgaI0Kv6kzGK' it should turn out this:

[
    {
        "length": 13,
        "payload": {
            "src": 819,
            "dst": 16383,
            "serial": 1,
            "dev_type": 6,
            "cmd": 6,
            "cmd_body": {
                "timestamp": 1688984021000
            }
        },
        "crc8": 138
    }
]

But I have an error:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb3 in position 1: invalid start byte

Also I have tried:

c = a.decode('utf-8') # UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb3 in position 1: invalid start byte
c = a.decode('utf-16') # UnicodeDecodeError: 'utf-16-le' codec can't decode byte 0x8a in position 14: truncated data
c = a.decode('ascii') # UnicodeDecodeError: 'ascii' codec can't decode byte 0xb3 in position 1: ordinal not in range(128)

But this is not working

The variable a contains:

b'\r\xb3\x06\xff\x7f\x01\x06\x06\x88\xd0\xab\xfa\x931\x8a'

Please help me decode this bytes I'm at a dead end