Methyl(trifluoromethyl)dioxirane (TFDO) in 1,1,1‐trifluoro‐2‐propanone (TFP)may be able to do your sought oxidation (see image  below): In there, cyclic ethers (shown example is tetrahydropyran) yield lactones by subsequent oxidation of a stable cyclic hemiacetal intermediate such as lactol in your case.
It is considered as selective and reactive oxidizing agent, capable of epoxidation of unreactive alkenes and arenes, oxyfunctionalization of alkanes, and oxidation of alcohols and ethers in following reference: Encyclopedia of Reagents for Organic Synthesis (1; March 2016); 11 pages. (https://doi.org/10.1002/047084289X.rm267.pub3)
You may also read: Oxidations by methyl(trifluoromethyl)dioxirane. 5. Conversion of alcohols into carbonyl compounds: J. Am. Chem. Soc., 1991, 113(6), 2205–2208 (DOI: 10.1021/ja00006a044).
Lactols are basically aldehydes in ring formation.
What happens is the lactols unwind in solution into chain formation and it is here that they are oxidized to carboxylic acid by something like Tollen's reagent. Then when they reform, they do so in lactone form.