possibility
(1) The load current is too small: your load current is not relatively small? The output ripple of this constant-voltage module will be relatively longer when the load is light. The routine test of all our products is measured under full load environment, you can consider slightly increasing the load and external filtering network,
(2) EMC requirements are high: if the noise requirements of the product are high: because the switching power module is realized by the switching oscillation circuit, the module itself will produce a certain noise, and the general LC filter network has no obvious effect on high-frequency noise. If you have very high requirements in this regard, it is best to connect the common mode choke at the output end of the module, see circuit application scheme. It can effectively suppress high-frequency noise interference, and it is also very effective for some occasions with high EMC requirements.
(3) You can also consider adding a 47pF-100pF safety capacitor between the input ground and the output ground of the module, but note that its voltage must be greater than the isolation voltage value of the module; 1000VDC product B0505LS-1W, its safety capacitor voltage should be ≥1000VDC.