Imagine that all of the Bitcoin network's hashing is being done on a single giant imaginary computer, that doesn't stop if it finds a successful block.
We can perform a calculation of the probability that it finds two valid blocks in one second using public estimates of the hashrate as well as the known target.
two_valid_blocks_in_1sec_probability ~= 100% * (hash_rate_per_sec * target / 2**256 ) ^ 2
Now imagine that the hash rate is distributed across the network of miners. Whether a valid block fails is dependent on the time it takes a valid block to reach, and be accepted by, rival miners. As an estimate, we can use the time it takes a block to reach nodes covering half of network hashrate; the block propagation time. Adjusting the above formula, it becomes:
valid_block_failure_probability ~= 100% * (hash_rate_per_sec * mean_block_propagation_time * target / 2**256 ) ^ 2
Note this assumes terms for ^3, ^4, ^5 ... (which, strictly, should be added on) are not significant. This also assumes that the time it takes nodes to validate and accept valid headers is << the mean propagation time.
Here's a python script to do that:
# Block 809295
hash_rate_per_sec = 405000000.0 * 10**12
# approximated from https://www.blockchain.com/explorer/charts/hash-rate
target = 0x00000000000000000004ed7f0000000000000000000000000000000000000000
# from https://learnmeabitcoin.com/technical/target
period_secs = 0.5
# from https://bitcoin.stackexchange.com/a/114196/34190
# mean block propagation time (a block will reach nodes responsible for half of network hashrate in this time).
prob_per_period = hash_rate_per_sec * period_secs * target / 2**256
N = 2
prob_N_hashes_in_period = prob_per_period**N
print('The probability that the bitcoin network will produce',N,'valid blocks in', period_secs, 'seconds is ', prob_N_hashes_in_period, ' Based on hashrate and target at block 809295.')
Which gives the following printout:
The probability that the bitcoin network will produce 2 valid blocks in 0.5 seconds is 6.81306666296744e-07 Based on hashrate and target at block 809295.
Using this, in answer to your question, we can say that, if the mean block propagation time is 0.5s, at block height 809295:
If a miner has found a successful block, there is a 0.00007% (1 sig fig) chance that it will not reach the consensus Bitcoin blockchain
The probability is sensitive to the estimated propagation delay:
valid_block_failure_probability [%, 1 sig fig]
The idea of collusion amongst miners (tacit or overt gatekeeping) slowing acceptance was not considered.