If you don't have a Form 6198 from 2018, you don't have "at-risk losses" that carry from 2018. That would be consistent with your statement that you always check the "all my investment in this activity is at risk" box in prior years.
Form 6198 reports and tracks losses from your partnership that you could not deduct in a year because the losses exceeded the amount you had "at risk" for the investment.
If you believe somehow that you had amounts that were not at risk in prior years, you would need to amend those prior year tax returns and limit your losses to the amount at risk in that year. Form 6198 would track these "at-risk disallowed losses" and carry them forward to future years when they could be deducted.
If you are uncertain, ask the partnership.
See the section of IRS Publication 925 at this link for the rules on at-risk limits.
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