Short answer: Under ordinary reimbursement, yes: the plan pays the claim and then files for reimbursement. Options such as specific advance funding (coordinated reimbursement) and monthly aggregate accommodation can ease that cash-flow strain, if the policy includes them.
Cash flow is a real risk in self-funding. A single transplant or NICU stay can require the plan to pay a very large amount before the insurer reimburses it.
Specific advance funding, often called coordinated reimbursement, is designed for extremely high-dollar claims: the carrier helps fund the claim payment up front so the employer is not carrying the whole amount while waiting for reimbursement. How it works varies by carrier, and it may need to be chosen at sale or renewal. Monthly aggregate accommodation reimburses claims over the monthly aggregate deductible during the year instead of waiting until year end, followed by a year-end reconciliation.
Confirm when funds arrive, what documentation is required, and whether an advance can be clawed back. A budget should show three numbers: expected cost, the contract maximum, and the cash needed in a bad month.